This invention relates generally to a process for preparing polymers. In particular, this invention relates to a process for controlling monomer levels in an emulsion polymerization reaction.
Polymerization reactions are complex chemical processes that require monitoring throughout the course of the reaction to ensure the desired polymer product is obtained. Traditionally, such monitoring was accomplished manually, either by analyzing the final polymer product and adjusting the necessary parameters for the next reaction batch or by taking samples during polymerization and analyzing them by any of a variety of techniques, such as by gas-liquid chromatography. However, it typically takes between 3 and 20 minutes to complete an analysis on a sample taken during the polymerization. By the time such results are obtained, it is typically too late to correct any problems with the polymerization and the entire batch may be ruined.
A solution to this problem is a real-time, on-line analysis of the course of the reaction. Typically, such on-line analyses monitor the heat generated during the course of the reaction and compare this number to a theoretical value. This difference is used to adjust such process parameters as temperature of the reaction, the amount of polymerization initiator added, the amount of monomer added and the like. For example, U.S. Pat. No. 4,742,472 (Sugimori et al.) discloses a method of controlling the temperature of an emulsion polymerization, thus resulting in a polymer having a uniform particle size.
European Patent Application 486 262 A (Baily et al.) discloses a method of adjusting the feeds of reactants or reaction temperature in order to move the polymerization reaction toward a theoretical model of the best course of the reaction, and preventing a buildup of unreacted monomer, thus increasing the safety of the reaction.
Gordon et al., Control of Particle Size Distribution Through Emulsifier Metering Based on Rate of Conversion, Emulsion Polymers and Emulsion Polymerization, American Chemical Society Symposium Series, 515–532, 1981, discloses that particle size distribution in an emulsion polymerization can be controlled by controlling the amount of emulsifier present at all times during the polymerization. In this article, the emulsifier is added to the polymerization reaction separately from the monomers.
However, known on-line polymerization analyses and control methods do not completely control the extent of polymerization in the early stages of a polymerization reaction. For example, such methods do not adequately account for the cooling medium present in a reactor cooling jacket at the initiation of polymerization, resulting in a large spike in the reaction. Such large spike is far from the true heat transfer rate from the reactor contents when the cooling medium is first turned on.
Wu in, Dynamic Thermal Analyzer for Monitoring Batch Processes, Chem. Eng. Prog., 81, pages 57–61 (1985) discloses using material/energy balance as a way of monitoring chemical reactions. This article does not disclose a method of adequately accounting for the cooling medium present in a reactor cooling jacket at the initiation of polymerization.
There is thus a continuing need for on-line reaction monitoring methods that provide real-time analyses of the reaction and provide accurate heat transfer rates from the reactor contents to the cooling medium when the flow of the cooling medium is first started, i.e. at the initiation of the polymerization reaction.